Is it good for a 12 year old to do a cranberry cleanse and is there any defects to it?

First of all, 12 year-olds should not be "cleansing" themselves. This is a time when children are growing and should consume a variety of fruits, vegetables, protein foods, etc.

Cranberries will give your child diarrhea, eliminating necessary electrolytes and cause dehydration. You will not be doing your child any good on a cranberry diet.